AuthenticationSuccessHandler and LoginController in Spring MVC

I have a made a user login interface which gets authenticated using Spring Security.

I made an authenticationsuccesshandler which redirects user to other page. Here is the Java Code that handles it.

protected String determineTargetUrl(final Authentication authentication) {
    boolean isUser = false;
    boolean isAdmin = false;
    final Collection<? extends GrantedAuthority> authorities = authentication.getAuthorities();
    for (final GrantedAuthority grantedAuthority : authorities) {
        if (grantedAuthority.getAuthority().equals("ROLE_USER")) {
            isUser = true;
            break;
        } else if (grantedAuthority.getAuthority().equals("ROLE_ADMIN")) {
            isAdmin = true;
            break;
        }
    }

    if (isUser) {
        return "/static_htm.html";
    } else if (isAdmin) {
        return "/console.html";
    } else {
        throw new IllegalStateException();
    }
}

I can't seem to understand if I wanted to add a Controller for displaying username for example or displaying error message on wrong login, how should I achieve it with or without a controller.

I also tried adding a new Java Package having Controller class but it didn't work. Any Ideas ?

Also, what is the difference between authenticationsuccesshandler and a controller ?